Inspection Procedure 4: The Brake Warning Light Does Not Illuminate When Ignition Switch Is in the ON Position (Engine Stopped)

Inspection Procedure 4: The brake warning light does not illuminate when ignition switch is turned to the ON position (engine stopped).

CAUTION
^ Whenever the ECU is replaced, ensure that the CAN bus lines are normal.
^ When the hydraulic unit (integrated with ASC-ECU) is replaced, always perform the calibration of steering wheel sensor, G and yaw rate sensor, brake fluid pressure sensor, cut valve, and inlet valve.

CIRCUIT OPERATION
^ ASC-ECU sends the illumination request signal of the brake warning light to the combination meter through ETACS-ECU via CAN communication.
^ When the ignition switch is turned ON, ASC-ECU illuminates the brake warning light via ETACS-ECU for approximately 3 seconds for the purpose of bulb check.

TECHNICAL DESCRIPTION (COMMENT)
In the case of this trouble symptom, there may be a malfunction to the CAN bus line, ETACS-ECU, combination meter, or ASC-ECU.

PROBABLE CAUSES
^ Damaged wiring harness and connectors
^ Malfunction of ETACS-ECU
^ Combination meter malfunction
^ ASC-ECU malfunction

DIAGNOSIS

Required Special Tools:
^ MB991958 Scan Tool (M.U.T.-III Sub Assembly)
^ MB991824: Vehicle Communication Interface (V.C.I.)
^ MB991827 M.U.T.-III USB Cable
^ MB991910 M.U.T.-III Main Harness

STEP 1. Using scan tool MB991958, diagnose the CAN bus lines.
Using scan tool MB991958, diagnose the CAN bus lines.

Q: Is the check result normal?
YES: Go to Step 2.
NO: Repair the CAN bus lines. Then go to Step 7.

STEP 2. Scan tool diagnostic trouble code
Using scan tool, check the diagnostic trouble code for ASC system.

Q: Is any DTC set?
YES: Carry out the diagnosis for the DTC.
NO: Go to Step 3.

STEP 3. M.U.T.-III actuator test
Perform the following actuator tests from the combination meter system to check if the brake warning light is illuminated.
^ Item 07: Indicator 1

Q: Is the check result normal?
YES: Go to Step 4.
NO: Carry out the diagnosis of the combination meter. Then go to Step 7.

STEP 4. Scan tool diagnostic trouble code for other systems
Using scan tool MB991958, check if diagnostic trouble code No. U0141 is set by the combination meter system.

Q: Is the DTC set?
YES: Diagnose the combination meter. Then go to Step 7.
NO: Go to Step 5.

STEP 5. Scan tool diagnostic trouble code for other systems
Using scan tool MB991958, check if diagnostic trouble code No. U0121 is set by ETACS system.

Q: Is the DTC set?
YES: Diagnose the ETACS-ECU. Then go to Step 7.
NO: Go to Step 6.

STEP 6. Retest the system.

Q: Does the brake warning light turn ON and OFF normally?
YES: Intermittent malfunction.
NO: Replace the hydraulic unit (incorporates in ASC-ECU). Then go to Step 7.

STEP 7. Retest the system.

Q: Does the brake warning light turn ON and OFF normally?
YES: The procedure is complete.
NO: Return to Step 1.